The present invention relates to a camshaft apparatus which is driven to rotate in synchronism with rotation of an engine, and more particularly to a camshaft apparatus which is supported rotatably by rolling bearings.
Generally, as an example of a bearing construction for supporting a camshaft rotatably in a shaft case, there has been known a slide bearing construction in which an outer circumferential surface of a camshaft 102 is supported directly by slide bearings 104 of a shaft case 140 as shown in FIG. 4. In this case, however, since the outer circumferential surface of the camshaft 102 is brought into direct contact with the bearing portions 104, there is caused a problem of short life or generation of vibration. In addition, as is shown in FIG. 5, in the slide bearing 104, when the camshaft 102 stops rotating, the camshaft 102 falls to rest on the bearing side and is then brought into line contact with the slide bearing 104 to thereby increase friction therebetween. This then causes a problem that the increased friction will results in a remarkable torque increase when the engine is started or is running at low speeds. Further, since the slide bearing 104 permits the axial relative slide of the camshaft 102, a restriction flange 102f is formed at an end portion of the camshaft 102 which lies at an end to which a pulley 109 is attached in such a manner as to be brought into sliding contact with the shaft case 140, so as to restrict the axial movement of the camshaft 102. However, in this bearing construction, the sliding friction between the restriction flange 102f and the shaft case 140 becomes large, leading to a drawback that the torque loss becomes large.
With a view to avoiding the problems, JP-A-2006-226183 discloses a camshaft apparatus in which the slide bearings are replaced by deep groove ball bearings. By the adoption of the deep groove ball bearings, the rotational sliding friction of the camshaft is changed from sliding friction to rolling friction, thereby making it possible not only to reduce vibration but also to extend the life thereof. Further, even when the camshaft stops rotating, since the periphery of the camshaft is surrounded by balls, the camshaft does not fall, thereby making it possible to reduce the starting torque. Further, in the deep groove ball bearing, since the inner ring and the outer ring are fixedly fitted in the outer circumferential surface of the camshaft and the inner circumferential surface of the shaft case and the balls constituting rolling elements roll in such a state that the balls fit in the groove-shaped raceway surfaces of the inner ring and the outer ring, a function to restrict the axial movement of the camshaft 102 is provided. As a result, the pulley-side restriction flange which slide contacts the shaft case can be eliminated from the camshaft, this also contributing to reduction in rotational sliding friction of the camshaft.
In recent years, with a view to reducing the weight of an engine and its peripheral portions, a configuration has widely been adopted in which a shaft case is made of light metal such as an aluminum alloy. On the other hand, since the camshaft is a component used in a power transmission system, the camshaft is made of a steel material in order to ensure strength and durability. However, the periphery of the engine where the camshaft is disposed tends to be heated to high temperatures and is frequently subjected to thermal cycling occurring in association with start and stop of the engine. In this case, there exists a large difference in linear expansion coefficient between the shaft case to which the outer rings of the ball bearings are fixed and the steel material to which the inner rings of the ball bearings are fixed. Due to the portion where the camshaft is disposed being heated to high temperatures and subjected to thermal cycling, the axial relative displacement of the camshaft relative to the shaft case becomes large considerably. However, since the plurality of bearings which support the camshaft are made up of the ball bearings, the thermal relative displacement between the camshaft and the shaft case is retrained strongly. As a result, an undesirable strong preload is imparted to the ball bearings in the axial direction, whereby an increase in torque tends to be called for easily due to an increase in rotational sliding resistance, and an eccentric wear of the raceway surfaces tends to be produced easily.